JagerBaBomb t1_iqwz0a9 wrote
Reply to comment by Mike__O in After DART: Using the first full-scale test of a kinetic impactor to inform a future planetary defense mission by EricFromOuterSpace
Shouldn't this be a simple matter of physics, though? Like, wouldn't the mathematicians already have the answer?
Edit: I greatly appreciate the educational responses!
jamin_g t1_iqx19xh wrote
Lots of assumptions as to the composition of the thing we're hitting.
The impact into a bowling ball vs an exercise ball is going to be very different.
Khazahk t1_iqx5ixm wrote
Well for one thing the exercise ball would be slightly easier to hit size wise.
Next question.
SmugBoxer t1_iqxf2g0 wrote
No need sir, you've been promoted to Head Mathematician. We're lucky to have access to a man of your caliber and scrutiny.
jeffh4 t1_iqxn4zm wrote
More like the impact into a loose pile of irregular boulders. The last couple of photos were stunning and unexpected. No smooth surface visible. Just jagged and loosely interlocked rocks.
Back to the chalkboard, folks! We need to explore the real possibility the probe emerged out the back of the asteroid without losing all of its momentum.
evanc3 t1_iqy56gx wrote
What do you mean? It looks extremely similar to all of the other asteroids that we've landed on/ photographed recently...
Mediumcomputer t1_iqz3705 wrote
Could one make a shaped charge or cone shaped nuke? I feel like that would deflect a pile of barely held together rocks better than our own Little Rock with solar panels on it
ciroluiro t1_iqy8evy wrote
I would imagine that the composition would be less of a concern than knowing the exact mass of the asteroid.
A collision in space is as good of a scenario you could get in terms of applying conservation of momentum. Then, knowing that the worst case scenario is a plastic collision (because the probe going through the asteroid completely is very much impossible), you could know with very good certainty the final momentum (assuming we have fairly precise and accurate information on the velocity of both objects). I'm purely speculating as I haven't looked into the mission, but I also doubt the scientist expected a best case scenario of a bounce from the asteroid.
VolvoRacerNumber5 t1_iqywdop wrote
I think everyone here is ignoring how material gets ejected by the impact. They know the momentum of both the spacecraft and asteroid very well. There's nothing new the impact will show in regards to that very basic momentum conservation problem.
What they are actually trying to characterize is the way debris is ejected by the impact. This ejecta carries off its own momentum and can impart a significant additional momentum to the asteroid.
ciroluiro t1_iqz47cw wrote
But that's kinda my point. Anything ejected would only "push off" the asteroid and help nudge it off course, given that they mostly get ejected either in the opposite direction to the spacecraft, or perpendicularly in a roughly even manner (this last one is speculation on my part since I imagine it depends heavily on the angle of impact and possibly more things I don't know, but I don't think it's a wild assumption)
Impiryo t1_iqywgoc wrote
There was a huge amount of ejected mass from the collision - that is likely the majority of the momentum change, and what we are measuring.
Plastic vs elastic collisions fail when explodey stuff happens.
ciroluiro t1_iqz3jul wrote
Unless the ejected mass went somehow forward, I don't see how that would steal momentum from the asteroid. Maybe if the ejected mass went perpendicular in a manner was not even in all directions? Kinda stretching it.
Anyway, explodey stuff would only help, not hurt.
Mike__O t1_iqx0lop wrote
Maybe, maybe not. I'm not a mathematician, physicist, or anyone in a position to know more than what I read online. My understanding is there are a LOT of variables that we don't necessarily entirely understand. The composition of the target is a big one, as well as density, uniformity of density (is one part of the target denser than the other, etc) and a bunch of other stuff. Not all space rocks are created equal, and it seems like the more we learn about these things the more we realize just how different they can be from one to the next.
We absolutely can calculate how much force one object of known mass and velocity can impart on another object of known mass and velocity, but when it comes to hitting space rocks there's a lot on the target side we simply don't know.
And even then, each situation will be different. If an object is a big enough threat to merit launching an intercept mission, it's likely that it will be too big to launch something at it capable of completely destroying it. Deflection will be the name of the game. Anyone who has played billiards will understand the critical role that impact angle plays when it comes to deflecting an object in the right direction.
coriolis7 t1_iqxlvrt wrote
No, because the impact is not an ideal collision. There is material ejected, which influences how much the momentum of the body changes. We have a rough idea of how much the velocity should change, but it’s still unknown how much, like on the order of tens of percent compared to theoretical.
If we underestimate how much of an effect an impactor has, we may expend far more resources or take more risks than are necessary to avoid a collision with Earth. Overestimate and we either get hit, or we have to send a second mission which may require an even larger deflection than the original mission (due to a shorter amount of time for the velocity variations to add up to a planetary miss).
sumelar t1_iqxsgte wrote
It doesn't matter how good the math looks, you always test.
That's how we ended up with relativity. Classical physics works great at a human level, but it's wrong at the cosmic and quantum levels, and testing proved that.
ericthefred t1_iqy2y0t wrote
Depends. Can you fill in the following unknowns for the mathematicians?
- How much energy went into changing the spin of the asteroid rather than its orbit
- How much energy was redirected sideways by the potential initial collision with a non-perpendicular surface?
- How much energy went into blasting loose material in all directions like a cue ball hitting a racked set of pool balls, multiplied by how many thousands or millions or billions for the unknown number of pool balls in the equation?
Mathematicians need to have data to work with.
SaltineFiend t1_iqyepz0 wrote
Also an object which ejects mass in space is known as a rocket engine. That ejecta plume was a rocket engine. It's a nonlinear equation that you have to run in real time to get a result. At the very instant you collide the two masses together the mass of the larger object changes.
ericthefred t1_iqz4qg1 wrote
An excellent point. When dealing with rockets, you have to account for the changing mass of the vehicle as propellant is exhausted. In order to integrate it into my list and remove the overlap with item 3 (being the part involving the energy in the ejecta), I would phrase it as: 4) How much mass has departed the asteroid?
Crowbrah_ t1_iqzmjae wrote
At least a kilogram, but I could be wrong.
Rowenstin t1_ir0lywt wrote
> Shouldn't this be a simple matter of physics, though? Like, wouldn't the mathematicians already have the answer?
If it was a perfectly inelastic collision of two rigid bodies, it'd be child's play. In reality, it's neither.
WirelesslyWired t1_iqyids4 wrote
Yes, Newton's laws of motion still hold, although the three body problem is hard to solve. Dimorphos, Didymos, and Dart make for an interesting computation.
The thing is, an asteroid is not a rigid body. How much of DART's inertia affected the speed of Dimorphos, and how much of it just knocked pieces of Dimorphos off of the main asteroid, and how all of this affected the speeds of both Dimorphos and Didymos is something that is complicated enough that it needs to have that calculation tested.
WhalesVirginia t1_iqz3f2q wrote
We have to wait to see how the orbit is modified after many cycles to get real numbers.
Prediction was like a 2% change after however many cycles.
So they don't even have the all the data yet to analyze.
UnnervingS t1_iqzgd5i wrote
The energy impacting the asteriod doesn't directly translate to change in velocity. The problem is accounting for heat, ejected particles, ECT. Not a mathematician btw
Bensemus t1_iqzo2vh wrote
Measuring stuff in the real world has error bars and reducing those error bars takes time.
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